"Let's make sure that history
never forgets the name... Enterprise." (Capt. Picard,
TNG: "Yesterday's Enterprise")

The Series

TNG: We'll Always Have
ParisThis episode is full
of "small" paradoxes where the characters encounter
themselves in moments that are only a few seconds or minutes in
the past or the future. The problem is basically the same
as with the grandfather paradox, although the impact is of course
less severe.Classification: random time shifts, without visible consequences

TNG: Time SquaredIn this episode Picard 2 is the only one to
survive the explosion of the Enterprise. He is thrown six
hours to the past together with his shuttlepod. Aboard the still
existing Enterprise in the past, Picard 2 suffers from a severe
shock; he initially cannot talk and doesn't remember anything. Moreover, the shuttle's
computer data cannot be read out. It is clear that this is primarily a ploy to keep the situation obscure.
Anyway, the effects shown here are doubtful with regard to other time travel episodes. The
difficulty with the shuttle's energy signature may be rationalized with
some technobabble. Picard 2's corresponding physical and mental
condition, however, seems to be directly related to the current
time, as if there were a temporal signature of his organism that
makes him feel better the closer he is to the time of his departure (ironically,
only to vanish the very moment that marks his departure to the past). This
doesn't make much sense, considering that whatever is responsible for biological and
electronic systems being out of phase here, the phase shift remains the same as long as they remain in the wrong,
earlier time frame. Moreover, there is no such effect in other episodes, only a weak
similarity to TOS: "All Our Yesterdays".

It is ethically highly questionable for our present Picard 1 to kill his
ill-fated counterpart from the future. It would have been absolutely sufficient to hold him back, since "the cycle must
end", as Picard 1 notes himself. Ironically, after changing the timeline in this
direction, Picard 2 disappears anyway together with the shuttle. There is an obvious paradox when the Enterprise does not explode, since in this case
there is no Picard to travel back in time and warn the
Enterprise.Classification: broken predestined timeline ("causality loop"),
with biological
impact of time travel

TNG: Yesterday's
EnterpriseWhen the
Enterprise-C appears from the space-time rift, the timeline from
the instant of its departure in the year 2344 to the present is immediately switched. It can be supposed that the Klingons regarded the
disappearance of the Enterprise-C as cowardice, which finally led
to the war timeline. Of course the Enterprise-D crew, as almost
all other beings in the galaxy, are unaware of the change, even as
Worf is replaced by Tasha. It remains a mystery how Guinan is
able to perceive the old timeline which has not been
"simply" erased from biological and computer memories,
but has actually never existed. No one would expect a biological
entity to have such a supernatural ability which is absolutely
useless for its normal life. One would rather suppose that beings
such as Q who move in more than three dimensions can perceive and
also manipulate timelines. Maybe Guinan is not the humanoid she
appears to be.

Apart from the temporal sequence, there is
also a logical sequence of events.

1.The Enterprise-C defends the
Klingon outpost and is finally destroyed, which is honored by the
Klingons (timeline 1).

2. The Enterprise-C is "pulled" into the
future (i.e. 2366) prior to being destroyed, which infuriates the
Klingons and creates the war timeline 2. The space-time
rift is not a static phenomenon, but originates at event 2, since
until then the old timeline 1 definitely existed.

3.The Enterprise-C is
"pushed" into the past again. This recovers event 1,
but not exactly, since Tasha is aboard the Enterprise-C.

4. The Enterprise-C defends the Klingon outpost and is
finally destroyed, which is honored by the Klingons. Some crew
members, among them Tasha, are "saved" by the Romulans,
only to be killed later on. However, Tasha gives birth to her
daughter Sela, who will appear later in TNG: "Redemption" and
"Unification" (timeline 3).

The problem with this episode is the fact
that the temporary timeline 2 should have been eliminated by
sending back the Enterprise-C to the past. This would not allow
Tasha to stay aboard the Enterprise-C, she would simply disappear
at the instant when the future is changed again, and Sela could
never exist. Vice versa, Captain Garrett, who died in a never
existing future, would come to life again (for a short time).
This is an obvious paradox. Perhaps the Enterprise-C, returning from the future, is shielded in some fashion so it would not be returned
to the state in which it left the year 2344.Classification: past incursion caused by accidental travel to
the future, remaining paradox (Tasha and Sela)

TNG: Captain's HolidayTime travel is only a secondary aspect of
the story, although the Tox Uthat is assumed to come from the
future. However, this is one of the few episodes where a
predestined timeline is established, since the two Vorgon time
travelers obviously anticipated that Picard would destroy the Tox
Uthat.Classification: predestined past incursion ("causality loop")

TNG: A Matter of Time No paradox occurs in the episode itself. However, by
leaving his own time which is in fact the 22nd century, Rasmussen
has probably already altered history. On a side note, it does not become
clear why Rasmussen, who has a timeship from the 26th century,
chose the less advanced 24th century to steal technical
equipment. Moreover, for an ordinary thief he seems to be too
well prepared, since he knows or pretends to know a lot of
details about the Enterprise. As much as he has learned about it, there should have been much easier
ways for him to get hold of valuable equipment than on the well-guarded
starship.Classification: intentional past/future incursion, without visible
consequences

TNG: Cause and Effect This episode features two different kinds of time
travel. First, the USS Bozeman unintentionally travels 90
years to the future, just to collide with the Enterprise. Second,
the Enterprise is caught in some kind of time loop. Every time
the Enterprise is destroyed, the present time for ship and crew
and probably the surrounding space region returns to an instant
some time before the ship's destruction from an external
viewpoint. In other words, the Enterprise and her crew do not
travel back in time, but time itself is not continuous within a
small region of space. This is corroborated by the statement that the
Federation time signal is 17.4 days ahead at the end of the episode, meaning
that the Enterprise has stayed in the time loop for this long as seen from an
external observer.

The phenomena in this episode can't be explained in plain
language. The starting point of the events seems to
be a space-time rift, potentially similar to the one in TNG:
"Yesterday's Enterprise". The Bozeman is pushed or
pulled into the future through this rift and almost immediately
collides with the Enterprise. The explosion of the Enterprise,
probably in connection with the surrounding space-time
distortion, causes the time reset. We do not know if the Bozeman
explodes as well or is directly affected by the destruction of
the Enterprise. However, it is obvious that the Soyuz-class ship
is caught in the time loop as well, since the collision occurs
repeatedly. Each time the Enterprise is restored to the state
that was a few hours ago, and the same must apply to the rest of
the space region. Otherwise, in the second and following laps the
crew would have registered discontinuities in sensor
measurements. We can assume that a Federation time signal is not
continuously received, although this was possible in TOS:
"The Naked Time". However, in any case the changes of
remote constellations are definitely measurable and should have
revealed the time reset. Since the Bozeman was not in the
respective space region some hours ago, the ship is thrown back
90 years after each explosion; only this way it can emerge from the rift each
time.

The story is substantially consistent if
we accept the unlikely fact that humanoids have a built-in sixth
sense for temporal discontinuities (déjà-vu) that Data as well
as the advanced subspace-physics based equipment of the
Enterprise do not detect. There is no paradox, since the repeated
collisions and explosions actually happened. Still, the chance of
a collision of the two starships is literally astronomical, even
if we take into account that the Enterprise is somehow attracted
by the space-time distortion.Classification: time travel to the future & broken causality loop in an
isolated region of space-time

TNG: Time's ArrowThere are two clear indications of a
predestined timeline in this episode. First, Data's head is found
in the cave before Data actually travels back to the past and
loses his head. Second, Guinan pretends to know Picard for a long
time, which is only confirmed "later" when Picard actually meets her
in the past. If we assume that back in the past the crew create exactly the timeline that has always
existed, whether they want it or not, this episode is self-consistent. Moreover, complicated
explanations of switching between different timelines are not necessary, since there is only
one timeline.

Nevertheless, the concept of predestination remains hard to accept
because it is an offense against free will. What if Data had stayed away from the scene under all circumstances as
Picard has originally planned it, in order to avert his time travel? Who could have hindered him to change history?
Likewise, what if Picard had taken Data's head with him when he returned to the
24th century?

Moreover, we have contradictory statements about
the mechanism of time travel in this episode. After his head has been found in
the cave, indicating that he will die, Data says: "There is no way anyone can prevent it, sir. At some future date, I will be transported back to nineteenth century Earth, where I will die. It has occurred. It will occur."
He thereby explicitly states that the timeline is predetermined, in contrast to
what happens in most of Star Trek's time travel episodes. Conversely, Guinan
claims that there is nothing like predestination. She insists on Picard joining
the away team, so she can meet him in the past. If everything were predestined,
there would be no need for her to take any influence with her knowledge about
the timeline. She also tells Riker on the topic of what he could do to save
Picard who has stayed behind in the past: "If I told you what happened in that cavern, it would affect any decision you'd make now. I can't do that. I won't."Classification: predestined past incursion ("causality loop")

TNG: TapestryQ may have forged the whole scenario much like a holodeck
simulation. However, if we believe Q, then Picard is given a second chance to avoid the
confrontation with the Nausicaans, in the course of which he lost
his natural heart. Q sends him back to his early days in
Starfleet, briefly prior to the incident.

Picard perceives himself as the aged captain, while everyone else sees the young Ensign Picard. At the end of the
episode, when Picard is revived (for which he is rather indebted
to Beverly than to Q), it is still not clear whether his
experiences in the past were true. We might tend to believe Q,
who has never been so honest and sympathetic than in this episode
and will never be again. In this case, the past has been slightly
changed, taking into account Picard's short affair with his
crewmate Marta.

Side note The authors missed a great opportunity in
connection with the TNG episode "Bloodlines", where Dai
Mon Bok alters a boy genetically so that he appears to be
Picard's son. The boy, however, could have been Picard's and
Marta's real son and this could have proved that Picard really
traveled to the past.

Q's promise that Picard can change his
own history without affecting other people is odd in any case: "Since you attach so much importance to the continuity of time, I will give you my personal guarantee that nothing you do here will end up hurting anyone, or have an adverse affect on what you know of as history."
In order to elude
the butterfly effect, Q would have to "filter" Picard's actions with
selective disregard for interdependencies and fabricate a history that could not
exist on its own. Q's demonstration that Picard, at his current age, would still
be a lieutenant junior grade instead of captain, only because he has always been
cautious, is doubtful and it insinuates that there is something wrong about the
whole scenario.Classification: multiple intentional past incursions that possibly never took
place, without visible
consequences

TNG: TimescapeThis episode features various time levels
in which time proceeds at different speeds. The time on and
around the Enterprise and the Romulan warbird almost stands still relative to the time experienced by Picard, Deanna, Geordi
and Data in the runabout. The concept and effect of time levels
is similar as in TOS: "Wink of an Eye", however, the
explanation is substantially different in the TNG episode. Here,
the time levels are a physical phenomenon, and the effects of
interaction between different time levels have to be considered
on this level. There should be physical as well as biological
problems for a character entering another time level. Yet,
only the latter ones have are partially shown in the episode.

Further investigation of the implications shows that
any interaction with a significantly different time level is
virtually impossible. Firstly, light or any other form of energy coming from a slower time
level is shifted to extremely low frequencies and also its intensity is
significantly reduced. Geordi can hardly locate the frozen Enterprise because "energy levels
are practically non-existent", yet it is possible to see everything
like it were on a normal level. Realistically, even Geordi would not see the
extreme infrared light. This is due to the simple fact that one corresponding
oscillation period is many times longer in the slow time level than in
the fast level. Secondly, the same applies to molecular motion, so
the temperature would be very close to absolute zero for persons from the fast
time level, and of course extremely uncomfortable. Thirdly, it is doubtful
whether it is possible to touch or move objects in the
slow time level or to breathe the molecules of the "slow" air, let alone
to cope with their low temperatures. Finally, even if interaction
is possible, for the slow computer it should take a long time until
anything is displayed. The writers could have avoided at least
the latter inconsistency by somewhat slowing down the LCARS response times.Classification: different time levels

TNG: ParallelsThis is one of the most fascinating Star
Trek stories ever. It focuses on parallel realities,
while the time travel at the end of the episode is only a
secondary aspect. The time travel probably serves to return to the
original timeline in "our" universe that would
otherwise have been altered by Worf's temporary absence, or
another Worf's presence.Classification: travel to another reality and back in time, without visible
consequences

TNG: FirstbornWe get no idea if and how the old Alexander's appearance in the
past would affect his own time and himself. The grandfather paradox is quite
evident, considering that young Alexander would be warned and probably wouldn't
travel back in time in his later years.Classification: past incursion, with unknown consequences

TNG: All Good ThingsQ is well-known for his fancies, and of
course it is him who arranges the weirdest time travel of Star
Trek so far. Maybe he takes pleasure in sending Picard on time
travels since TNG: "Tapestry". Firstly, three different
times in the past, the present and the future are involved, and Q
arbitrarily transfers Picard from one time to another. Secondly,
Picard is transferred into himself, i.e. he replaces his alter
ego in the past and the future. In a "normal" time
travel there would be two simultaneous Picards. Thirdly, his
actions in the past do not have any bearing on the future, the
three times are isolated from each other with respect to the
normal forward course of time. Finally, vice versa, the captain's actions
in the future do affect the past. It seems that Q employs any
possible trick to bring about the anti-time reaction that destroys all
mankind in the past, only to blame Picard for it. Picard is only
a marionette in Q's highly unfair fancy, however, despite all off
Q's attempted deception, the best Star Trek captain ever does his
job.

The fact that the Enterprise is destroyed
in "Star Trek Generations" might be a sign that
the TNG episode does not describe an actual future, but only a
future in Q's imagination. However, in DS9: "The
Visitor" another possible future is shown, and the future
Defiant crew wear the same uniforms and badges as on the future
Enterprise and Pasteur. Apart from the obvious budget savings in the real world,
this is a sign that Q's future is not inevitably the real future,
but at least a realistic one.Classification: multiple travels to multiple times, corrected or without visible
consequences

The Movies

Star Trek VII:
GenerationsThe movie features the Nexus, a phenomenon that allows either to stay in
the wonderful world inside or to travel anywhere any time. In
other words, the Nexus can do everything. This quality is
certainly not beneficial for the story, as it leaves too many
options and too many questions. According to the Star Trek
Encyclopedia II, Soran was only briefly inside the Nexus in 2293 until he was "rescued" by the Enterprise-B transporter
beam. This assumption makes more sense than the theory that the
El-Aurians were expelled from the Nexus by the Borg. Exactly the
same as to Soran must apply to Guinan. However, while Soran seems to be outside the Nexus when Picard meets Kirk, there is a representation of Guinan
left in the Nexus and able to talk to Picard.

Picard and Kirk probably chose the worst
possible instant to leave the Nexus and return to reality when
Soran already had his finger on the launch button. If it is true
that the two could have traveled anywhere any time, they could
have returned e.g. to the Amargosa Observatory. Captain Picard
probably considered possible effects on the timeline, however,
either he wanted to prevent the destruction of the inhabited
solar system under all circumstances or not. In particular, Kirk
could have chosen a better time for himself to return to reality. What if
Kirk had gone back to his own time and probably changed history?

In this movie a paradox does not occur,
because the Nexus probably works as a kind of buffer between the
timelines. The Nexus is obviously a domain outside of our
space-time, and is therefore not affected by changes in our universe. So the old timeline, in which the
Veridian system and the
Enterprise saucer are destroyed, ceases to exist. However, Picard
is nevertheless always present in the Nexus (like Guinan), and so
he is able to leave the Nexus and fight Soran in the new
timeline.Classification: past incursion to change history, successful

Star Trek VIII: First
ContactWhen the Borg alter history by
sabotaging Cochrane's first warp flight in 2063, the
Enterprise-E is protected from the change, since the ship is
already in the vortex that leads back in time. So far, the
authors have learned from errors in the past when such a phenomenon almost
customarily remained
unexplained. When the Enterprise-E
arrives in the year 2063, it is already too late to prevent the formation of a
new timeline, since the Borg have begun to shoot at
the launch site and have killed a number of people that might
have been important for history. Picard's first idea should have
been to travel back about one more day (using the slingshot effect) and immediately destroy
the Borg sphere as soon as it arrives. Even without the Borg's
attempt to capture the Enterprise, the success of the actual efforts to correct history
is questionable. It seems to have worked though, and the
Federation is the same as before, or has only imperceptibly
changed.

We may want to speculate that "First Contact"
features a predestination since Riker quotes the future Cochrane
and Cochrane might have the quote from Riker (not to mention that Cochrane may
have been inspired to name the first Warp-5 ship "Enterprise").

However, why did Geordi recreate the Borg
vortex to return to the future, instead of choosing the good
old slingshot effect, which is even possible with a primitive
Bird-of-Prey? It is possible that the missing deflector didn't allow to go to
warp, but it was never mentioned as being so crucial, so it must have been obvious to the crew that
the Borg vortex was the only or the best option to go back.

A problem arises with regard to VOY:
"Year of Hell", where Seven of Nine knows about the
Phoenix because "the Borg were present during those
events". The first explanation could be that some Borg
were still alive, or at least their subspace transmitters were functional when the Enterprise arrived back in the 24th century,
and they contacted the Collective. Secondly, they could have used
temporal transmitters such as in VOY: "Timeless". Thirdly, although the modification of the sensor
dish failed, a conventional subspace signal could have made it to
Borg territory in the 21st century. However, in the two latter
cases the Borg would have been warned, and this should have
altered the timeline...Classification: past incursion to change history, corrected